I. Field of the Invention
This invention relates to a method of growing crystals from melts, and more particularly to a method of growing multiple monocrystalline layers.
II. Description of Prior Art
A multi-layer structural semiconductor element wherein a semiconductor monocrystalline layer is formed on a monocrystalline insulating substrate, such as SOS (silicon on sapphire) is widely applied to a semiconductor device such as a MOS transistor or a junction transistor. Further, a semiconductor element consisting of multiple monocrystalline layers of different semiconductor materials is also applied to various devices such as, for example, a surface wave device utilizing the respective characteristics of the semiconductor materials constituting the monocrystalline layers.
A prior art method for producing such layers comprises growing on a monocrystalline substrate previously prepared by a well known technique such as Czochralski method a desired monocrystalline layer using a crystal growth method including a gas-phase or liquid-phase growth method, or a melt growth method. For example, U.S. Pat. 3,650,822 discloses a method for producing multiple monocrystalline layers by a melt growth which comprises immersing an electrically insulating substrate body in an appropriate semiconductor melt and slowly pulling the substrate body from the melt at a rate of 50 to 100 mm/hr to permit the semiconductor to be epitaxially grown on the surface of the substrate body. This method can produce such multiple layers as the aforesaid SOS.
The above-mentioned prior art method necessitates two independent operations -- an operation for obtaining a monocrystal constituting a substrate and an operation for further growing a monocrystal on the substrate thus obtained, and its overall process is troublesome and its mass-productivity is low. Further, the prior art method would impose a limitation upon the substrate size, and the size of a multi-layer structural semiconductor element as finally obtained, accordingly, is limited to that of the substrate. Further, in such a method as disclosed in the above U.S. Pat. No. 3,650,822, the substrate, particularly a portion of the substrate at the side of the melt stays in the melt for a relatively long time, and therefore the heat of the melt remains in the substrate for a relatively long time even after the substrate is withdrawn from the melt. As a result, the mutual diffusion of atoms from either of the substrate or the crystalline layer into the other takes place, rendering the boundary between layers unclear.